Magnetometer consisting of two sensors with means for unbalancing each sensor at null condition

ABSTRACT

A highly sensitive magnetometer consisting of at least one pair of generally rectangular core elements having square hysteresis loops is disclosed. The primary windings of each of the cores are driven by an oscillator, with the secondary windings of each core being differentially connected through a null circuit, which serves to balance the effects of background fields, and a detector to a suitable indicator. The primary windings of the cores are operated in a resonance and the secondary windings are arranged to provide a high null output which eliminates the dead band caused by the threshold value of the diodes in the output detector circuit. This high null output may be accomplished by unbalancing the core windings or by feeding a portion of the primary winding drive current, or a signal synchronous therewith, to the corresponding secondary winding, thereby permitting measurement of the small magnetic fields which previously fell within the dead band of the device. Also disclosed is a method of aerial surveying and a guidance system for off-vertical drilling using the magnetometer.

United States Patent Schad [54] MAGNETOMETER CONSISTING OF TWO SENSORSWITH MEANS FOR UNBALANCING EACH SENSOR AT 51 Oct. 24, 1972 OTHERPUBLICATIONS Geyger; W., Flux Gate Magnetometer Uses Toroidal Corp.Electronics; June, 1962, pp. 48- $2 NULL CONDITION Ling; S., FluxgateMagnetometer For Space Applica- 72 Inventor: Charles A. Schad, Tulsa,Okla. gg iif ffgg [73] Assignee: Kalium Chemicals Limited, Regina,

Saskatchewan, Canada Primary Examiner-Roben J. Corcoran [22] Filed: July21, Attorney.lones and Lockwood PP 164,553 [57] ABSTRACT Related US.Application Data A highly sensitive magnetometer consisting of at least[63] Continuatiomimpan of Ser No 791 039 Jan one pair of generallyrectangular core elements having 1 4 1969 abandoned square hysteresisloops is disclosed. The primary windings of each of the cores are drivenby an oscillas21 us. c1. ..324/43 R with semndary windings each we being[51] 1111. C1. ..G0lr 33/04 diffmmiauy hmugh a Circuit whim [58] Fieldof Search........324/43 R, 47, 4, 8; 340/197 serves to balance theeffects of background fields, and a detector to a suitable indicator.The primary [56] References Cit d windings of the cores are operated ina resonance and the secondary windings are arranged to provide a highUNITED STATES PATENTS null output which eliminates the dead band causedby 2,476,273 7/1949 Beach ..324/43 R the threshold value of the diodesin the output detec- 2,560,132 7/1951 Schmitt .324/43 R tor circuit Thishigh null output y be accomplished 2,942,180 6/ 1960 Coker ..324/43 R yunbalancing the core windings or by feeding a por- 3,445,928 5/1969Beynon....................324/43 R tion of e P y d g drive cur ent, or asignal 3,449,665 6/1969 Geyger ........................324/47synchronous therewith, to the Corresponding seconda- 3,484,683 12/1969Wong ..324/43 R ry winding, thereby permitting measurement of the smallmagnetic fields which previously fell within the FOREIGN PATENTS 0RAPPLICATIONS dead band of the device. Also disclosed is a method of611,194 12/1960 Canada ..324/43 R aerial surveying and a guidance systemfor off-vertical drilling using the magnetometer.

34 Claims, 11 Drawing Figures i Nl JLL J, -i:on I Q: S3253 3C: 5:93:1en; e 11, c

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sum 1 OF 9 INVENTOR CHARLES A. SCHAD PAIENTEDIIBI 24 I972 SHEEI 3 (If 9hwN mucjmsz PATENTEDHM 24 I972 SHEEI 5 BF 9 b b \k mm PC3015 UZEPm JJEQPATENTEflnm 24 um SHEEI 7 [IF 9 PATENTED um 24 m2 SHEEI 8 BF 9 FROM 360giom 401 F ROM 364 FTROM 37a MAGNE'IOMETER CONSISTING OF TWO SENSORSWITH MEANS FOR UNBALANCING EACH SENSOR AT NULL CONDITION CROSS REFERENCETO RELATED APPLICATION This application is a continuation-in-part ofprior copending application Ser. No. 791,039, filed Jan. 14, 1969, andnow abandoned and entitled Magnetic Detection and Magnetometer SystemTherefor.

BACKGROUND OF THE INVENTION The present invention relates, in general,to magnetometers and their associated circuitry, and more particularlyto a highly sensitive system for measuring or detecting magnetic fieldsin mineral exploration,

nondestructive magnetic testing, guidance systems for off-vertical welldrilling and other related applications where a capability for measuringminute variations in magnetic fields is required.

Although magnetometers are well known in the prior art, and theirutility in the measurement of various magnetic characteristics has beenestablished, the sensitivities of such prior devices have beeninsufficient to permit successful application in weak field conditions.Thus, their usefulness has been limited to conditions where relativelystrong magnetic fields are present, and the prior art does not teach howsuch devices may be made to have the sensitivities required for accurateuse in the various activities mentioned above. Thus, for example, in theuse of magnetometers in guidance systems for off-vertical drillingequipment, where a target magnetic field source is located in one drillhole and the drill to be guided is located in an adjacent well hole,with means to guide the drill toward the target, it has been determinedthat in order to obtain the required directional accuracy, a sensitivityof 0.05 gammas would be required. Since in a well-hole environment thehorizontal component of the earth's magnetic field is between 14,000 and28,000 gammas within the United States, it is apparent that the accuratemeasurement of the relatively small field that would be generated by atarget solenoid requires an extremely sensitive magnetometer,particularly where the distance between the wells is great.

Again, where airborne exploration is to be conducted by measuring theminute variations in the earths magnetic field caused by the presence ofmineral deposits underground, extreme sensitivity is required ifaccurate measurement and mapping is to be accomplished. Thenondestructive magnetic testing of manufactured articles to locatedefects in the materials used or in joints formed in the materials isbecoming more and more common, and with increased usage, increasedsensitivities are required for more accurate measurements.

In general, two major systems have been employed by the prior art in themeasurement of magnetic characteristics by magnetometers. The firstgeneral system is the provision of circuitry for feeding back a signalwhich is used to null the field being sensed. When cancellation of thefield is obtained, the system is balanced and a measurement of themagnitude of the feedback signal required is indicative of the strengthof the field being measured. The second system, which is most commonlyused in the prior art where high sensitivity is required, utilizes asecond-harmonic signal which is derived from the sensor windings throughselective filtering. The phase and amplitude of this second-harmonicsignal is detected by comparison to a standard double-frequencyreference, with the output being indicative of the direction andmagnitude of the sensed field. After amplification and phasedomodulation, this output could be displayed or recorded. A third,optically pumped, system may also be employed in certain instances, butis not suitable for many purposes in spite of its high absoluteaccuracy.

Little success was obtained in utilizing prior art magnetometers in therange of sensitivities provided by the present invention. It was foundthat the prior devices functioned well enough in strong fields, but indetecting fields of less than 1 gamma the normal background noise andthe drift of the circuit components masked the signal being measured andresulted in an unintelligible display. These problems were particularlyapparent in phase-sensitive systems, where relatively high noise tosignal levels at the sensitivities employed by the present system madethem unusable.

It has been usual in prior magnetometer systems to measure the strengthand direction of a magnetic field through the use of two magnetic coreswhich are mutually perpendicular to provide 360 direction sensing bygiving both X and Y coordinate vectors. The vectors are commonlyresolved electronically and displayed as an azimuth on a cathode raytube, for example. Such displays are compensated to eliminate theeffects of external magnetic fields to thereby permit measurement of theselected magnetic characteristic. However, the use of electronicaddition of vectors to provide a direct resultant display is notsuitable for the sensitivities at which the present invention operates,and the normal noise and drift in the earths magnetic field preventseffective nulling of the external fields. Further, since themagnetometer can assume any position relative to magnetic north, thenoise components which affect the magnetic cores are almost never thesame, and since they are not sufficiently small to be ignored, constantreadjustment of the prior systems becomes a major, time-consumingproblem.

Various other restrictive factors are involved in providing amagnetometer capable of operating at extremely sensitive levels, for atsuch levels environmental conditions, cable transmissioncharacteristics, variations in cable impedence when different lengths ofcable are used, variations in components, and the like can becomecritical, and it becomes necessary to compensate for or eliminate theeffects of these various factors. Yet each compensation and eachfiltering step serves to reduce the sensitivity, and thus the accuracyand reliability, of the prior art devices. For the foregoing reasons,and others, prior art magnetometers were found to be unable to produceusable results in the measurement and detection of very weak magneticfields, and thus were considered to be unsatisfactory.

SUMMARY OF THE INVENTION resolving magnetic fields at the level of 0.01gammas, or 10" Oersteds.

It is a further object of the present invention to provide a highsensitivity magnetometer that produces easily identifiable outputsignals at sensitivities well beyond those available with prior devices.

Another object of the invention is the provision of a magnetometersystem which may be used in conjunction with a wide variety of magneticfield measurement activities.

Thus, it is an object of the present invention to provide a method ofaerial surveying through the use of a highly sensitive magnetometerwhich will permit more accurate aerial prospecting for mineral deposits.

It is a further object of the invention to provide a method ofoff-vertical well drilling which utilizes the highly sensitivemagnetometer of the present invention and which thus enables moreaccurate drilling operations.

It is another object of the present invention to provide a magnetometerdevice and associated operating circuitry which will provide a highlysensitive magnetometer capable of detecting minute magnetic fields andwhich thus may be used in such diverse areas as the location of oredeposits, guidance systems for underground drilling operations, locationof metallic objects underground or underwater, measurement of magneticpolarization for use in subsequent degaussing applications,nondestructive testing, and various other activities, in each case thesensitivity of the present device producing a greater capability andgreater degree of accuracy in various applications than is possible withprior art devices.

An additional object of the invention is the provision of a method ofoperating a magnetometer which will provide improved accuracy andsensitivity over the prior art.

The foregoing objectives are accomplished through the use of an improvedmagnetic structure for the magnetometer device as well as through theuse of improved circuitry in conjunction with such structure. Twomagnetic core elements, each having a closed, generally rectangularconfiguration, are arranged, in a preferred embodiment, to be mutuallyperpendicular and in parallel planes, one core being located above theother. An energizing winding and a secondary, or pickup winding, isprovided for each core, with each winding consisting of two coils, onewound on each leg of the core. The primary winding induces in thesecondary winding signals which are coincident and 180 out of phase,under balanced conditions, so that no output results. Imposition of anexternal field alternately advances and retards the time phase ofsaturation of the two legs of the core so that a differential outputsignal results at the secondary winding. The output signal from eachsecondary winding is proportional to the vector component of the imposedfield which is parallel to the axis of the corresponding core. Such anexternal field will affect both of the perpendicularly mounted cores,thereby providing vector signals which may be resolved to provide anindication of the direction and magnitude of the external field in theplanes of the cores. Where the two cores are perpendicular, resolutionof the vector components is accomplished in conventional manner usingrectangular coordinates, as by plotting the component amplitudes andsolving graphically for the actual field direction in the plane of thecores. In this configuration the magnetometer has a uniform sensitivity,and for this reason it is generally preferred.

if desired, the magnetometer cores may be arranged with the core axes atother than a right angle, in which case the directional sensitivitybecomes nonuniform; as the two axes approach parallelism, thedirectional sensitivity approaches that of a single core. When the coresare not mutually perpendicular, the resolution of the output signals toobtain the actual direction and amplitude of the magnetic field becomesmore complex, but it can be accomplished by known mathematicalprocedures or by a graphical resolution using a nonrectangular system ofcoordinates. 1n the latter case, the measured components of a magneticfield H from each of two cores A and B are plotted on the nonrectangularcoordinates, the A core measuring a component H cosa and the B coremeasuring a component H cosa where a is the angle between the field Hand the axis of core A, and a, is the angle between the field H and theaxis of core B. lt will then be seen that the intersection of H sine,and H sina will determine the location of the plotted line representingthe direction and amplitude of the field H.

A nonperpendicular arrangement of cores has limited use, however,because as the angle between the cores decreases the signals to beresolved can become smaller. For example, where the cores areperpendicular at least one of the signals will always be equal to orgreater than 0.707H, and the smallest that both can be at the same timeis also 0.707H. On the other hand, if the cores are arranged at a 45angle, the same minimum signal value becomes 0.38211. Thus, where smallfields having unknown directions are to be measured, it generally isdesirable to arrange the cores to be mutually perpendicular so thatmaximum signal is obtained.

The use of two cores permits measurement of that component of theexternal magnetic field of interest which lies in the plane of thecores. By taking measurements of this external field with the cores inhorizontal and vertical orientations, the absolute direction andmagnitude of the field can be determined. Again, if the cores aremutually perpendicular and the two orientations are perpendicular, theresolution of the measured field components can be carried out in theconventional manner, whereas nonperpendicular measurements require morecomplex mathematical or graphical manipulation. Although the presentinvention is described in terms of two cores, it will be apparent that athird sensor with its associated circuitry may be used in combinationwith the two described cores to permit measurement of three magneticfield components. This arrangement allows calculation of the absolutedirection and amplitude without the need for reorientation of the cores.Again, it would be conventional to arrange the three cores to bemutually perpendicular, although nonperpendicular arrangements could beused.

The magnetic cores preferably are of a material exhibiting a generallysquare or rectangular hysteresis loop, and thus are normally driven intosaturation during the operation of the device. The primary windings aredriven by a suitable oscillator to provide the required alternatingcurrent, and a null balancing means is connected to the secondarywindings to balance out the effect of any magnetic fields which are notof interest in the measurement being made. A portion of the exitationcurrent driving each primary winding is fed to the secondary winding inorder to insure that the null output from the secondary windings has anamplitude sufficient to overcome the conduction threshold of thedetector means which is connected to each secondary winding. To furtherincrease the level of the null output, capacitors are connected acrossthe primary windings to provide a resonant condition in the primarywindings. The detector means connected to each of the secondary windingsincludes a diode which serves to convert the output signals to acorresponding direct current, and it is to these diodes that the portionof the excitation current is applied to insure conduction even at a nullcondition and thus to eliminate the loss of signal normally caused bythe threshold of conduction of such devices. To improve the accuracy andreliability of the magnetometer measurements, the output of eachsecondary winding is monitored separately, the outputs being alternatelyswitched into common amplifiers and recorders to eliminate the errorsthat are inherent in systems which utilize a separate amplifier for eachchannel. Such errors arise from inherent differences in thecharacteristics of amplifiers, such as nonlinearity, drift due totemperature changes, and the like.

In addition to the foregoing, or as an alternative, conduction of thedetector diodes can be insured at null conditions by intentionallyunbalancing the primary windings by a slight amount so that one side ofthe secondary always receives a larger pulse than the other side. Thenet effect is to provide an output equal to the difference between thetwo sides which is sufficient to produce a null output that willovercome the diode threshold so that any superimposed outputs caused bythe measured field will be passed by the diodes. A similar effect can beobtained by selective positioning of the secondary winding on the core,without altering the coil symmetry. It appears that such positioning ofthe core causes a differential output under external field nullconditions because of discontinuities in the core caused by, forexample, changes in permeability or in the amount of core material atpoints where the tape wound core starts and finishes. However, becauseof the difficulty in obtaining an optimum location for the secondary,this latter method is not very practical.

The null balancing means consists of a selectively variable source ofdirect current which may be connected to the secondary windings in orderto compensate for the effects of undesired external magnetic fields. Inorder to prevent this null balancing current from affecting the detectorcircuits, coupling capacitors are connected between the secondarywindings and the detectors. The null balancing current then is feddirectly to the secondary windings to compensate for the earths magneticfield or other background fields.

The magnetometer described above, and operated in conjunction with theabove-described circuitry, exhibits a sensitivity which enables it to beused in a large number of applications in the place of existingmagnetometers to provide an accuracy and reliability not previouslyavailable. One such application is illustrated in detail in the presentapplication, wherein there is provided a guidance system foroff-vertical well drilling. In this system, the magnetometer describedhereinabove is located in a target well, and a magnetic field generator,such as an electromagnet, is located in a second well some distance fromthe first. The electromagnet, or solenoid, is carried by a drill stringwhich is to be guided in accordance with the measurements of thegenerated field at the target well, as obtained by the magnetometer.These measurements provide an indication of the direction of thegenerated field and slight changes in the measured components willprovide an immediate warning of changes in the direction of travel ofthe drill with respect to the target magnetometer, whereby errors can becorrected and accurate control exercised. The sensitivity of themagnetometer of the present invention provides an improved detectioncapability, and this results in considerable savings in the time andexpense involved in the drilling of off-vertical holes.

The guidance system includes, in addition to the magnetometer describedabove, an alternating current supply for the drill string magnet whichprovides a unique and easily identifiable magnetic field. The AC supplyincludes a pair of cam driven potentiometers which provide a transientfree, square wave, AC waveform. Also included in the guidance system isa control motor for directional guidance of the drill, attitude sensorswhich permit remote monitoring of drill position, and various switchingnetworks which permit calibration of the monitoring system, activationof the sensors, and operation of the control magnet and directionalmotor, all in a predetermined sequence. A switch position indicatormechanism is also provided to permit monitoring of the switchingnetworks.

The described guidance system, utilizing the increased sensitivity ofthe target magnetometer of the present invention and the improvedcontrol and sensing mechanism of the drill string circuits, permits moreaccurate results than were available with prior magnetic guidancesystems, and thus permits off-vertical holes to be drilled more quickly,and with less expense than before. In addition, target and drill holescan be spaced further apart than was possible with prior devices.

BRIEF DESCRIPTION OF THE DRAWINGS Although the present invention isdescribed in terms of preferred embodiments, it will be understood bythose skilled in the art that various modifications of the specificcircuitry are embraced within the scope of the invention as defined bythe claims. The foregoing and additional objects, features andadvantages of the present device will best be understood and appreciatedfrom the following detailed description of a preferred embodiment of theinvention and of a specific application thereof which have been selectedfor purposes of illustration and are shown in the accompanying drawings,in which:

FIG. 1 is a diagrammatic illustration of magnetic core elements suitablefor use in the magnetometer of the present invention;

FIG. 2 illustrates the square hysteresis loop characteristic of themagnetic core of FIG. 1;

P16. 3 is a block diagram of the electric circuitry used in conjunctionwith the magnetic core of FIG. 1 to produce the highly sensitivemagnetometer of the present invention;

FIG. 4 is a schematic diagram of the base portion of the circuitry ofthe system of FIG. 3;

FIG. 5 is a schematic diagram of the remote portion of the circuitry ofthe system of FIG. 3;

FIG. 6 is a diagrammatic illustration of a guidance system foroff-vertical drilling;

FIG. 7 is a block diagram of the guidance system of FIG. 6;

FIGS. 8A-8B are a schematic diagram of the base portion of thecircuitryof FIG. 7;

FIG. 9 is a diagrammatic illustration of a waveform generator suitablefor use in the circuit of FIG. 8; and

FIG. 10 is a schematic diagram of the remote portion of the circuitry ofFIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning now to FIG. 1 of thedrawings, there is illustrated a pair of magnetic cores l0 and 12arranged at an angle to each other and lying in the same or in parallelplanes. In use, the cores preferably will be located one above theother, instead of side-by-side, but the arrangement of FIG. 1 provides abetter view of the manner in which the windings are arranged. As hasbeen explained, the angle between the axes of the cores preferably is90, so that each core will respond to the components of the magneticfield of interest which correspond to the X and Y axes of a conventionalrectangular coordinate system, as illustrated in FIG. I. If anon-uniform pattern of sensitivity is desired, the cores can be arrangedin various configurations, with the appropriate mathematical analysisbeing used to relate the core orientations and measured field componentvalues in order to obtain the desired information concerning the fieldof interest. For purposes of this disclosure, it will be assumed thatthe axes of the cores 10 are mutually perpendicular, and represent thecomponents of an external field measured along the X and Y and Y and Zaxes of a conventional rectangular coordinate system.

The cores are fabricated in conventional manner from a magnetic materialhaving a very nearly rectangular hysteresis curve, such as thatillustrated in FIG. 2 where the field (B) induced within the core isplotted against the external magnetizing field (H). Core 10 carries aprimary winding made up of coils l4 and 15 wound in the same directionon opposite legs of the core whereby the magnetic flux generated by acurrent I flowing in the primary winding is additive around the core.Similarly, core 12 carries a primary winding comprised of coils l6 and17 connected in series on opposite legs, producing a magnetic flux whichis additive.

A secondary winding is carried by core 10, and includes coils l8 and 19which are oppositely wound on the two legs of the core, whereby magneticflux in the core tends to induce oppositely flowing currents in eachsecondary coil. The signals induced in coils 18 and 19, under idealconditions with an equal number of turns in each coil, are coincidentand 180 out of phase, so that no output results. In similar manner, core12 carries coils 20 and 21 making up a differential secondary windingwhich, under ideal conditions, produces no output. An alternativearrangement for the secondacircuitry used in the 8 ry windings utilizesa single coil in place of coils l8, l9 and a single coil in place ofcoils 20, 21, the single winding in each case enclosing both legs of itsrespective core, so that the differential effect is retained. However,for convenience, reference will herein be made to the two-coilembodiment.

As the magnetic field due to the alternating current I in the primary ofeach core increases in a positive or negative direction, the magneticflux in the cores increases in a corresponding manner until a point isreached where no further change can occur in the flux with a furtherchange in the current, and the core is saturated. When a core materialhaving the square loop characteristic of FIG. 2 is used, the saturationpoint is reached very suddenly, as shown at plus or minus B... and uponreversal of the current, the saturation curve changes and follows adifferent path. Thus, when an alternating current which has sufiicientmagnitude to drive the core to saturation in both directions is appliedto the primary windings, the induced flux will follow the curve of FIG.2. Under balanced conditions no output will be obtained from thedifferentially wound secondary windings.

If a balanced core arrangement is exposed to an external magnetic fieldhaving a component H parallel to the axis of core 10 and a componentl-I, parallel to the axis of core 12, the time phase of saturation inthe two axial legs of cores l0 and 12 will be changed. If the fieldcomponent H is in the direction illustrated in FIG. 1, it will aid thefield H, produced by primary winding coil 14 and oppose the field Hproduced by the primary winding coil 15. This will result in an advancein the time phase of saturation of the upper leg 22 of core 10, whileretarding the saturation time of lower leg 23 by an amount proportionalto the strength of the component H The currents induced in secondarycoils l8 and 19 will no longer be equal and opposite, and an outputsignal proportional to the unbalance, and thus to the magnitude anddirection (polarity) of the field component l-I will be produced by thesecondary winding. In similar manner, the secondary coils 20 and 21 willproduce an output signal proportional in amplitude and polarity to themagnitude and direction of field component H,,. The two output signalsthus represent the X and Y coordinate vectors from which may be resolveda resultant vector indicative of the applied magnetic field. Themagnetic cores and their windings may thus be referred to as fluxvalves, flux gates, or magnetic sensor elements, 25 and 26,respectively.

Magnetometers operating in accordance with the foregoing principles arewell known in the art. The vectors obtained in the manner describedabove from the secondary windings of flux gates generally are combinedand displayed as an azimuth on a cathode ray tube, as a spot on such atube, or simply as a meter reading which may be plotted. However, theconside rations mentioned hereinabove have prevented such systems fromoperating in changing conditions with the required degree of sensitivityor reliability, and for these reasons the magnetometer circuitry of FIG.3 has been devised.

Referring now to FIG. 3, the sensors 25 and 26 are indicated in theirnormal relationship and are diagrammatically shown as supported by asuitable housing 30.

The type of housing in which the saturable core elements and theirassociated windings are mounted will depend upon the particular use towhich the magnetometer is put. However, a suitable housing would includeFormica side and top walls adapted to receive and cover the flux gates,with the coils and other wiring being potted or otherwise suitablesealed in a material such as silicon rubber. The sensors 25 and 26 willnormally be mounted in parallel, spaced horizontal planes, with the axesof the cores being mutually perpendicular, and in this position willmeasure horizontal components of magnetic fields. By rotating thesensors to lie in vertical planes or by providing a third sensor whichis perpendicular to sensors 25 and 26, the vertical components of thesame fields can be measured to permit determination of the absolutedirection and magnitude of the field.

As is well known in the art, the magnetometer housing may includesuitable means for properly orienting the cores, as will be furtherdescribed hereinbelow, so that meaningful measurements of an unknownfield can be obtained. In applications where the only parameter ofinterest is a deviation from a measured value, the orientation means cansimply be a stabilizer to hold the magnetometer in a fixed position. ifan absolute measure of an external field is needed, or if themagnetometer is to be moved, as in a well logging or surveyingoperation, it may be necessary to provide a more sophisticatedorientation means such as a gyro arrangement or a servo system. However,such mechanisms do not form a part of the present invention.

The primary windings of the magnetic cores 10 and 12 in the sensors aredriven by a power circuit which includes, as shown in FIG. 3, a powersupply 32 energized from a suitable source of alternating current 34through a power switch 36. The output of the power supply is fed by wayof transformer 38 to a constant current supply 40 and thence by way ofterminals F to a voltage regulator 42 to energize a free runningoscillator 44. The oscillator produces an alternating output ofpredetermined frequency and magnitude which is fed through a bufferamplifier 46 and a pair of driver amplifiers 48 and 50 to feed theprimary windings of cores l and 12, respectively. Terminals F arerepresented as being a part of a connector 52 which may provide a directconnection between the two terminals or which may be a suitable cable orother link for remote location of the sensors with respect to the powersupply. By connecting the power supply on one side of the connector 52and the voltage regulator and oscillator on the other side of theconnector, the system can accommodate long lengths of cable between therespective terminals without adversely affecting the sensitivity andaccuracy of the magnetometer. The particular nature of the connector 52will depend, then, upon the particular application of the magnetometersystem, as will be seen more clearly from the detailed descriptionshereinbelow.

The secondary windings of cores l0 and 12 are connected separately tocorresponding channels in a null circuit 54. The null circuit isenergized by means of a null balancing circuit 56 which, in theillustrated embodiment, receives its power from power supply 32 by wayof transformer 58. Any output from the null balancing circuit 56 isconnected through a selector switch 60, either terminal D or terminal Cin connector 52, and through line 62 or 63 to the null circuit, aselected null balancing current is applied to the secondary windings ofeach sensor. Selector switch 60 allows the output from the secondarywinding of either core or core 12 to be fed through its correspondingchannel in a detector 64 to corresponding terminals A or B in connector52 and thence to equalizer 66. The desired channel output from equalizer66 is selected by selector switch 60, whereby the sensor output beingdetected is fed to an amplifier and range selector 68. The amplifiedsignal may then be displayed on a meter 70 and recorded on a suitablerecording means 72. The recorder, which is energized from the powersupply 32 may, for example, be a conventional multi-pen device wherein aseparate pen is driven by each sensor output.

The selector switch may be manually operated to permit extendedmeasurement of one or another of the sensor outputs, or may be driven toprovide rapid sampling and substantially simultaneous display of the twooutputs. It will be apparent that either mechanical or solid-stateswitching may be used for switch 60, or for other switches illustratedherein.

The use of separate output channels for each of the two sensors allowsthe outputs to be monitored separately, whereby the accuracy andreliability of the measurements are improved. The separate outputs maybe individually recorded, and calibration curves may be used for eachsignal to provide compensation for any differences between the sensors.By feeding the signals from both secondary windings alternately throughthe same amplifier, variations in the amplifier will not appreciablyaffect the readings, since the relative magnitudes of the two signalsare more important than their absolute magnitudes. If two amplifierswere used, differences in their drift characteristics (i.e., due to temperature changes) would produce a differential gain which wouldintroduce error during the use of the equipment. With a singleamplifier, however, such drift will not alter the accuracy of the resultas long as this drift does not occur during the short interval betweenmeasurement of the two sensor outputs, and long term drift will notafiect direction sensing even if it alters the magnitude of thereadings.

An advantage of reading the output from each secondary windingindividually is that the actual wave form of the signals may berecorded. Then, even though the recording may drift or wander withbackground noise, it is possible to detect with considerable accuracy avery small signal superimposed on the noise, if the waveform of thesignal is known. Such a known signal waveform may be caused, forexample, by a generated magnetic field which varies in accordance with apredetermined pattern, which pattern can be discerned on the recordedsignal. If the two outputs are electronically added before therecording, it becomes much more difficult to distinguish the signalfield from the background noise. Recording the actual waveform offersthe further advantages that malfunctions in the system are simpler tospot, for changes in a received known waveform will be indicative of thepresence of and nature of such a malfunction. Further, fewer componentsare required for the system, thus increasing reliability.

in operation, then, the magnetometer system of P16. 3 utilizes a freerunning oscillator at the location of sensors 25 and 26 to provide theexcitation current to the primary windings of each sensor. The nullbalancing circuit 56 feeds a current by way of null circuit 54 to thesecondary of each sensor to balance out known external fields, such asthe earths magnetic field, in the region of the sensors whereby theoutput from each sensor will normally be zero. After the null conditionhas been obtained, a variation in the external field, such as may becaused by a mineral deposit or by a superimposed target field, willproduce a corresponding change in sensor output from the null balancecondition, resulting in a detectable differential output signal from oneor both sensors. The output signal from each sensor is applied through acorresponding channel in the equalizer 66 to the amplifier 68 forsubsequent display on a meter or recorder, or both. The coordinatevectors of the variation in the field being measured thus are detectedby the two sensors, and the resulting changes in the two outputs at themeter 70 and recorder 72 may be analyzed to obtain the magnitude anddirection of the field variation being sensed.

Turning now to a more detailed consideration of the magnetometer systemof FIG. 3, there is illustrated in FIG. 4 a schematic diagram of thecircuitry on the power supply side, or base portion, of connector 52.FIG. is a schematic diagram of the circuitry for the sensor side, orremotely locatable portion, of the connector 52 in FIG. 3. The variousblocks in the block diagram of FIG. 3 are illustrated in FIGS. 4 and 5by dotted lines similarly numbered.

Turning now to FIG. 4, the power supply 32 receives its input fromalternating current source 34 by way of power switch 36. The alternatingcurrent source may be filtered by means of capacitors 80 and 81connected between the two sides of the l 17 volt input and ground. Aneon indicator lamp 82 is located within the power supply 32 and lightswhen the power supply is turned on. A fan 83 may be provided, ifrequired, to maintain a flow of cooling air across the electroniccircuitry used in the present system. The input power is converted tothe desired voltage level by means of a constant voltage transformer 84connected across the AC source. The supply for recorder 72 is connectedacross the secondary of transformer 84 by way of recorder switch 85, asindicated at 86. A neon indicator lamp 87 is connected across therecorder supply 86 to indicate when the recorder is energized. 1f therecorder is a pen-type wherein permanent traces of the sensor outputsare recorded on a paper chart, a chart drive motor 88 may be providedfor the recorder and may be selectively energized by either push buttonswitch 89 or toggle switch 90, both of which are in series with recorderswitch 85 and connect the chart drive motor across the secondary oftransformer 84. Neon lamp 91 is connected across the chart drive motorto indicate when it is energized.

Also connected across the secondary of transformer 84 are the two powersupply output transformers 38 and 58. These output transformers areconnected in parallel with each other and in series with a magnetometerswitch 92 for controlling energization of the magnetometer circuitry. Aneon lamp 93 indicates whether the output transformers are energized.

Output transformer 38 feeds the constant current supply 40 which is acurrent-regulated rectifier circuit. The upper end of the secondary oftransformer 38 is connected to opposed diodes and 101 in the common fullwave rectifier arrangement. The outputs of the diodes are connectedthrough capacitors 102 and 103, respectively, to the lower end of thesecondary winding on transformer 38 to provide a filtered DC signalacross lines 104 and 105. The series arrangement of a Zener diode 106and resistor 107 is connected across lines 104 and 105, with thejunction between these elements connected to the base of a transistor()1 in line 105, the Zener diode providing a constant voltage at thebase of O1 to regulate the current flow therethrough. The emitter of O1is connected through resistor 108 to the other side of Zener diode 106,while the collector is grounded. Output terminal G is connected to thegrounded collector of Q1 while output terminal F is connected to line104 and carries the regulated DC current to connector 52.

Terminal F in FIG. 5 carries this regulated direct current to line 110in the remotely locatable portion of the magnetometer, which line thuscarries the 13+ supply for the oscillator 44, buffer amplifier 46 anddriver amplifiers 48 and 50. The voltage on line 110 is regulated at theremote location by voltage regulator 42 which comprises a Zener diode111 connected between line 110 and the ground line 112 connected toterminal H. A capacitor 113 is connected across diode 111 to provide afilter for any AC components that might appear in line 110 by reason ofthe conditions encountered in connector 52. By including the regulator111 at the remote portion of the magnetometer, the rated voltage on line110 is assured, regardless of the nature of the connection representedby connector 52.

Oscillator 44 is a conventional free running oscillator utilizingtransistors Q2 and Q3 connected in a known manner. The feedback pathsfor the transistors are parallel "T" networks, or notched filters, andthese feedback networks determine the frequency of the oscillatoroutput. Resistors 116 and 117 connect the collector-emitter circuit oftransistor Q2 between lines 110 and 112. The base of O2 is connectedthrough resistor 118 to line 112 and through resistor 119, the parallelT networks 120 and 121, and resistor 122 to the base of Q3. The junctionof the parallel T networks and resistor 122 is connected to thecollector of 03, through resistors 123 and 116 to line 110, and to theoscillator output line 124. The emitter of O3 is connected directly toground line 112. The oscillator is designed to run at about 2,000 Hz,the output signal being applied through line 124 to the buffer amplifier46 which comprises a transistor Q4 connected in an emitter-followerconfiguration. The signal on line 124 is applied through resistor 125 tothe base of 04, the collector being connected through resistor 126 tothe supply line 110 and the emitter being connected through resistors127 and 128 to ground line 112. The junction of resistors 127 and 128provides the buffer amplifier output, which appears on line 130.

The 2,000 Hz signal on line 130 is applied to the base electrodes ofdriver amplifier transistors 05 and Q6 by way of blocking capacitors 131and 132, respectively, in corresponding drivers 48 and 50. The emitterelectrodes of Q5 and 06 are connected to ground line 112 while therespective collectors are connected through resistors 133 and 134 tosupply line 110. Bias resistor 135 is connected between the base andcollector of Q while bias resistor 136 connects the collector of O6 toits base electrode. The output signal from driver 48 is derived from thecollector electrode of Q5 and is applied by way of line 140, seriesblocking capacitor 141 and shunt capacitor 142 to one end of the primarywinding of sensor 25. The other end of this primary winding is connectedby way of line 143 to ground. Similarly, the output from driveramplifier 50 is derived from the collector of transistor 06 and appliedby way of line 144 through series capacitor 145 and shunt capacitor 146to one side of the primary winding of sensor 26; the other side of thisprimary winding is connected by way of line 147 to ground line 112.Coupling capacitors 141 and 145 are used to connect the outputs of thedriver amplifiers to the sensors in order to eliminate the need for ironcore coupling transformers in the vicinity of the saturable cores and12. Capacitors 141 and 145 not only block unwanted direct current, butare used as impedance matching elements.

Capacitors 142 and 146 are shunted across the primary windings ofsensors 25 and 26, respectively, and are of such a value that the coresl0 and 12 are driven in a resonant condition. By operating these devicesin resonance, very high excitation, or magnetizing, currents aregenerated for very short intervals of time, without excessive heating ofthe core windings. Generally, the current flow in the primary windingsof the sensors is limited by the relatively high inductance of themagnetic core material; however, upon saturation of the core, thecurrent is limited only by the winding resistance, the circuitresistance and the air inductance of the winding. As a result,relatively high currents can flow in the primary winding while the coreis saturated. The shunt capacitor receives and stores energy from thedriver amplifier until the core saturates, at which time the chargecarried by the capacitor flows through the low impedance of the windingand is superimposed on the current from the driver. The capacitordischarges in a very short time, abruptly converting the energy of thecapacitor to electromagnetic energy in the field surrounding the primarywinding. When the capacitor has discharged, this field collapses tocharge the capacitor in the opposite direction and saturation in thereverse direction begins. The oscillator serves to replace the energylost in the various impedances and to provide the initial saturation ofthe core in each direction of the alternating input signal, and properselection of the capacitor causes a resonant flow in this LC circuit.

Since the amplitude of the signal induced in the secondary winding ofeach core is proportional to the time rate of change of the fluxgenerated by its corresponding primary winding, very little voltage isinduced into the secondary winding while the core is saturating.However, upon saturation of the core, a substantial pulse is induced inthe secondary during the high amplitude, short time pulse caused by thedischarge of the shunt capacitor. In one embodiment of the sensors,built in accordance with the present invention, a peak primary currentpulse in excess of two amperes was measured, this pulse having aduration of approximately eight microseconds. This represented a peakmagnetizing force of about forty Oersteds, or about 400 times the fieldrequired to saturate the core. Thus, very high output pulses may beinduced in the secondary windings of the cores, even though thesaturated cores cause the operation to be equivalent to that availablein air core transformers.

One side of the output, or secondary, winding of sensor 25 is connectedby way of line 150 to terminal E in connector 52. Reference to FIG. 4will show that terminal E is grounded in common with terminal H. Thissame side of the secondary winding for sensor 25 is also connected, byway of line 151 and through DC blocking capacitor 151', to terminal G inconnector 52, which terminal is also grounded. The other side of thesecondary winding for core 10 is connected by way of line 152 to a loadresistor 153 connected between lines 151 and 152. A loop comprisingcapacitor 154 and resistor 155 is connected in series between line 152and the input to the primary winding of sensor 25. This loop permits adirect coupling of a portion of the primary drive signal into thesecondary winding to increase the level of the secondary null conditionoutput, for reasons to be explained hereinbelow.

One side of the secondary winding on sensor 26 is connected to terminalE and, by way of lines 150 and 151 to terminal G. The other side of thissecondary winding is connected by way of line 156 through load resistor157 to line 151 and through capacitor 158 and resistor 159 to the inputof the primary winding on sensor 12. This RC circuit forms a loopwhereby a portion of the primary drive signal may be fed directly to thesecondary winding.

As has been noted above, under ideal conditions where no external fieldis present, a zero output will be obtained from the secondary windingsof each sensor 25, 26, because they are differentially wound; under suchideal conditions there will be no voltage across load resistors 153 and157. However, as a practical matter it is not possible to obtain thisideal condition and there will usually be some differential outputappearing on the secondary windings due to differences in the variouswindings, imperfections in the core material and, most particularly, byreason of external or ambient magnetic fields. Means are provided,therefore, to compensate for undesired outputs due to these variousfactors, and this is the function of null circuit 54 and null balancingcircuit 56. Of course, if a specific external field is to be measured,the null circuit is not used to compensate for it, but only tocompensate for undesired fields. Where variations from the normal valueare to be detected, or where specific field variations superimposed onthe earths magnetic field and other background fields are to bedetected, then the normal output due to the earth's magnetic field andother undesired fields must be eliminated. This is accomplished in thedisclosed preferred embodiment by means of the null balancing circuit 56indicated in FIG. 3 and shown in schematic form in FlG. 4.

Referring now to FIG. 4, null balancing circuit 56 is powered by meansof transformer 58, the secondary winding of which is grounded at itscenter tap, with the outer ends feeding a full wave rectifier 170. Thepositive side of the rectifier is fed through line 171 across filtercapacitor 172 to a first voltage regulator which consists of atransistor Q7 having its base connected through Zener diode 173 toground line 174. The emitter output of O7 is connected across shuntcapacitor 175 by way of line 176 to the input of a second series voltageregulator 08. The base electrode of Q8 is held at a predeterminedvoltage level by Zener diode 177, with the emitter output of thetransistor appearing across shunt capacitor 178 on line 179.

In similar manner, the negative output of full wave rectifier 170 isapplied by way of line 181 across capacitor 182 to the input of voltageregulator transistor 09. The base of O9 is connected through Zener diode183 to the common ground line 174. The output from O9 is applied acrosscapacitor 185 and is carried on line 186 to the input of transistorregulator Q10. Here again, the base of 010 is connected through Zenerdiode 183 to the common ground line 174. The output from Q9 is appliedacross capacitor 185 and is carried on line 186 to the input oftransistor regulator 010. Here again, the base of 010 is connectedthrough Zener diode 187 to common line 174 and the output of Q10 isapplied across capacitor 188 to output line 189.

The series regulators in the positive and negative output lines from thefull wave rectifier provide a very stable positive voltage on line 179and an equally stable negative voltage on line 189, which voltages areused to supply the compensating voltage for the two sensors 25 and 26.The variable voltage for sensor 25 is obtained from a pair ofpotentiometers 200 and 201 connected between positive and negative lines179 and 189. Potentiometer 200 provides a coarse adjustment of the DCvoltage to be applied to the sensor, while potentiometer 201 provides afine adjustment, both potentiometers being variable from a positive to anegative value, with their center positions representing a zero voltage.The slide wires of the two potentiometers are connected through suitableresistors to an output line 202 which is connected through a nullcircuit power switch 203 and line 204 to one arm 205 of selector switch60. When switch arm 205 is closed, the output voltage frompotentiometers 200 and 201 is applied to terminal D in connector 52.Referring now to FIG. 5, it will be seen that the voltage appearing atterminal D will be applied by way of line 62 through a resistor 210 tothe secondary winding of the saturable core in sensor 25. This inducesin the core a DC flux component which may be adjusted by variation ofpotentiometers 200 and 201 to a value having a magnitude and polaritywhich will effectively cancel the component of any undesired externalfield which is parallel to the axis of sensor 25. It will be noted thatcapacitor 154 prevents this DC voltage from reaching the primary ofsensor 25.

A second pair of potentiometers 220 and 221 are connected between lines179 and 189 to provide an output line 222 a DC voltage of selectedamplitude and polarity for application to the secondary winding ofsensor 26. The signal on line 222 is applied through null circuit powerswitch 203 and line 224 to a switch arm 225 on selector switch 60. Thenull circuit power switch, when closed, connects the null balancingcircuit to lines 204 and 224. At the same time, switch 203 connects lamp223 across the primary of transformer 38 to provide an indication of thestatus of the switch.

When switch arm 225 of selector 60 is closed, the output voltage frompotentiometers 220 and 221 is applied by way of terminal C and line 63(FIG. 5) through resistor 226 to line 156 and thence to the secondarywinding of sensor 26. Again, this DC voltage is blocked from the primaryof sensor 26 by means of capacitor 158. The DC signal supplied by way ofterminal C produces a flux in the core of sensor 26 which is of selectedpolarity and amplitude to permit external field components parallel tothe axis of the core to be balanced out so as not to affect its output.

In some applications, as where the earth's magnetic field is to becompensated, in order to detect other, localized, fields, it may bepossible to provide the required null current by the use of nullsensors. Such null sensors would be at a remote location, and wouldproduce output signals proportional to the fields to be compensated.These output signals could then be fed directly to the null circuit 54for application to the secondary windings in order to provide automaticcompensation for the ambient field. The null sensors would, in such anarrangement, be parallel to the cores which they are to compensate so asto provide the correct output current, but would have the advantage ofproviding an automatic response to changes in the undesired backgroundfield.

By proper adjustment of the null balancing circuits, background andother undesired magnetic effects can be cancelled from the output of thetwo sensors, allowing them to detect with great sensitivity changes inthe background field or varying magnetic fields superimposed thereon bya solenoid or the like. Such changes will appear as alternating currentson sensor output lines 152 and 156, which signals will pass throughcorresponding blocking, or decoupling capacitors 151', 230 and 231 andwill appear across shunt resistors 232 and 233, respectively. Theprovision of decoupling capacitors between the secondary windings andthe detector circuits permits injection of the null balancing currentdirectly into the secondary windings, without adverse effect on thedetector. Thus, the secondary windings perform the dual function ofproviding the output signal as well as compensating for undesiredambient or background magnetic fields, and eliminate the need for anadditional winding.

Terminals D and C are connected through capacitors 235 and 236 andcapacitors 237 and 238, respectively, to ground line 151. These filtercapacitors serve to prevent any AC components in the DC null balancingcurrents, which might be generated by variable cable loading or othernoise factors, from being applied to the secondary windings of thesensors. Furthermore, these capacitors shunt to ground any AC outputfrom the secondary windings which might flow back through lines 62 and63 to the null balancing circuit 56.

In order to maintain maximum reliability and sensitivity, the outputsignals from the sensors are not amplified in the remote portion of themagnetometer. Because of the extreme range of sensitivity of the presentdevice, remote amplification would require provision for rangeswitching. In applications where the remote and the base portions of thecircuit are close together or are constructed in a single unit, thisdoes not present any particular problem. However, where the remoteportion is to be located a considerable distance from the base portion,or in an inaccessible spot, a capability for range switching complicatesthe system unnecessarily, and reduces reliability. Further,

since the detected signals are being processed in separate channels inthe remote portion, a duplication of circuitry would be required whichwould introduce differential errors due to differing drift andtemperature characteristics. To avoid these problems, the outputvoltages appearing across resistors 232 and 233, representing theoutputs from sensors 25 and 26, respectively, are converted to directcurrent for transmission by way of terminals A and B to the base portionof the system. This is accomplished by applying the voltage acrossresistor 232 to the full wave detector rectifiers 240 and 241, producinga varying direct current at terminal A which is representative of thefield sensed by sensor 25. In similar manner, the voltage appearingacross resistor 233 is detected by full wave rectifiers 242 and 243 indetector 64, producing a varying direct current at terminal Brepresentative of the field component sensed by sensor 26.

Although the conversion of the detected signals to direct currentproduces numerous advantages, particularly where conductor 52 is a cablehaving considerable length, nevertheless such conversion to directcurrent without preamplification presents some problems. For example,near the null condition of the magnetometer there exists a dead" bandwhich is below the diode threshold voltage. This dead band requires thatthe output signal from a sensor build up to a certain point before itwill exceed the diode threshold voltage and produce substantial diodeconduction. Several features of the circuitry already described havebeen included to overcome this problem. For example, the use of aresonant driving condition for the primary windings of the sensorsprovides a strong signal output from the magnetometers. This, togetherwith the resistive loading of the secondary windings, the injection ofpart of the primary drive current into the secondary and other featuresto be described, insures that ample voltage will be available duringboth positive and negative pulses to exceed the diode threshold, andthereby provide the requisite sensitivity.

An additional method of increasing sensitivity is to slightly unbalancethe primary winding of each sensor intentionally so that one side of thesecondary always receives a larger pulse than the other. This may beaccomplished by providing a slightly higher turns ratio on one side ofthe primary than on the other. This unbalance does not materially affectthe time of the core saturation of the two legs of the sensor, butduring the saturated interval the structure is reduced essentially to anair core differential transformer having a higher turns ratio on oneside than on the other. The net effect is an output pulse proportionalto the unbalance, which output can be adjusted to produce a null fieldoutput of sufficient positive and negative pulse amplitudes to exceedthe diode thresholds. A preferred method of overcoming the detectorthreshold is to supply a portion of the primary drive signal fed to eachprimary winding to the corresponding secondary winding, as by RC network154, 155 for core and RC network 158, 159 for core 12. The amplitude ofthe drive signal so coupled to the secondary windings can be adjusted toproduce an alternating output when the external magnetic fields havebeen balanced to produce a null field condition in the cores, and thisoutput will be of suiticient amplitude to insure diode conduction. Themagnetic field of interest that is sensed by the sensors then issuperimposed on this portion of the primary drive signal and is detectedby the diode circuits. Another method of providing a null output is toinject into the secondary windings an external signal which issynchronous with the primary drive current, or to supply an isolateddirect current to the diodes in the detector to forward bias them. Thesevarious techniques may be used together in various combinations toeliminate the need for AC amplification and to increase the sensitivityof the magnetometer circuit, although the preferred method, which isillustrated herein, is the use of a circuit means to feed the primarycurrent to the secondary winding.

The signals on terminals A and B, representing the sensed magneticfield, are applied to the base unit for the magnetometer system (FIG.4), after passage through connector 52, and are fed to correspondingequalizer networks in equalizer 66. The equalizer for the signals onterminal A consists of a potentiometer 250 connected in series through aresistor 251 to ground line 252. The sliding arm output frompotentiometer 250 is applied by way of line 253 to one terminal ofswitch arm 254 which is a part of selector switch 60. In similar manner,terminal B is connected through potentiometer 256 and through resistor257 to ground line 252, with the sliding arm of the potentiometer beingconnected through line 258 to a second contact for switch arm 254.

Selector switch 60 permits determination of which of the two sensors isto be activated and applies to that unit the null balancing current fromits corresponding null balancing network, while at the same timeconnecting the corresponding output channel from the selected unitthrough its equalizer network to switch arm 254. From there the signalis fed through normally closed push button contact switch 260 and line261 to the amplifier and range selector 68. As has been noted, switch 60may be manually operated or automatically driven, with its switchingspeed being dependent upon the type of record which is to be made.

DC amplifier 265 is of any conventional design and therefore is notshown in detail. The positive supply voltage for this amplifier isobtained from the regulated positive voltage appearing on line 176 inthe null balancing circuit 56, while the negative supply for theamplifier is obtained by way of line 186 in that same circuit. The rangeselector is comprised of a variable RC shunt across amplifier 265 andincludes a capacitor 266 and a plurality of selectable shunt resistors.A movable switch arm 267 permits selection of any desired one of theshunt resistors, each of which may be adjusted to provide the desiredrange of response. This range selector arrangement thus regulates theamplification of DC amplifier 265, the output of which is applied toreadout meter and, by way of resistor 270 and potentiometer 271 to asuitable recorder 72. Selector switch 60 thus feeds the outputs of themagnetometer core elements selectively or alternately through a commonDC amplifier having selectable calibrated ranges and providing bothmeter and recorder outputs. The amplifier and the range selectorresistors may be calibrated through the use of variable resistor 275,which may be switched into the input of amplifier 265 by depressing pushbutton switch 260, and the amplifi- 19 er may be balanced by adjustmentof variable resistor 276.

The particular structural and circuit features of the magnetometer asdescribed above with respect to FIGS. 4 and 5 produce a device which isextremely sensitive to magnetic fields. A device made in accordance withthe foregoing description has functioned at a sensitivity or resolutionlevel of 0.01 gammas. With the horizontal component of the earthsmagnetic field ranging from 14,000 to 28,000 gammas within the UnitedStates, this resolution level permits measurement of magnetic fieldswhich have changes as small as 1 part in 2,800,000 with respect to theearths field. This degree of sensitivity provides a capability for usein many fields, as will be recognized by those skilled in the art. Inparticular, however, the magnetometer is useful in the measurement ofmagnetic characteristics which are weak with respect to the strongbackground field provided by the earth s magnetic field. Thus, itappears that the present invention will be of great value in airbornemagnetometer surveying.

One of the major problems in aerial surveying involves the accuratepositioning of the air craft carrying the measuring instruments so thatproper readings can be obtained. The extreme sensitivity of the presentmagnetometer substantially reduces this problem in that a determinationof a magnetic gradient can be made with a very small distance betweenadjacent measurements. In prior devices, it was necessary to fly two ormore parallel paths over the territory being surveyed in order to obtainreadings of the magnetic field which differed sufficiently to provide ameasurable gradient which could be plotted and used to indicate thepresence of mineral deposits or unusual formations. The paths had to besome distance apart because of a lack of sensitivity in the magnetometerunits used. As will be appreciated, maintenance of a predetermineddistance between the parallel paths was extremely difficult, and thusthe results were limited in accuracy. The present magnetometer overcomesthis problem because its sensitivity permits accurate measurement of afield gradient using magnetometers spaced only 8 or 10 feet apart. Thismakes it possible to obtain a measurable gradient in one pass over agiven area by mounting a pair of magnetometer sensors on a singlesupport which may be lowered out of an aircraft. The support allows thesensors to be far enough removed from the aircraft that degaussing ofthe plane is not necessary.

Photographs of the area to be surveyed may be obtained in stereoscopicpairs from the U. S. Department of Agriculture, and from thesephotographs contour maps may be made. A mosaic pattern may then be laidout on the map to form a grid defining the flight path of the aircraft.Orientation points are selected and during flight over the selected areaphotographs are taken concurrently with the magnetometer readings toprovide accurate location of the reading. At the same time, the altitudeof the magnetometer sensors is also recorded. The recorder and the otherbase portion circuitry of the magnetometers are located within theaircraft, the recorder providing a record of the magnetic readings, thelocations where photographs are taken, and the height of the plane. Thespaced sensors measure different magnetic patterns, and this pattern, or

gradient, provides information concerning the earths geologicalstructure. From this information, data may be obtained concerning, forexample, the location of mineral deposits, as indicated by variations inthe normal value of the earth's magnetic field, and by characteristicgradient patterns that can be more accurately measured in a single passover the territory with a fixed distance between sensors than waspossible with plural passes where the distance between the magnetometerpaths varied in an unpredictable manner.

The magnetometer of the present invention also finds an important use inconjunction with a system for guiding an off-vertical drill. Theguidance of off-vertical drills, and more particularly the drilling of ahorizontal hole between a first vertical hole and a second target holeis generally taught in the prior art. See, for example, US. Pat. No.3,285,350 and US. Pat. No. 3,406,766, both of which have issued to .l.K. Henderson. These patents describe a system wherein a signal-sendingdevice is positioned in one of the holes and a signal receiving deviceis positioned in the other well. The sending device is mounted on thedrilling mechanism, and means are provided for controlling from thesurface the direction of the drill in response to indicating signalsfrom the receiver. The magnetometer of the present invention can beadapted for use as the receiver in such guidance systems, and thesensors for such a receiver are indicated in FIG. 6 at 300. The receiver300 corresponds to the remote portion of the present magnetometer asillustrated in FIG. 5, but is mounted in a nonmagnetic housing which,for example, may be stainless steel. The receiver is gimbal mounted andlead weighted so that it is self-leveling to insure that the sensors arein a horizontal position so that accurate measurements can be obtained.

The receiver 300 includes the sensors 25 and 26, the oscillator voltageregulator, the oscillator, the buffer amplifier and drivers, the nullingcircuitry and the sensors for the magnetometer system, as describedhereinabove. A suitable guide mechanism 302 locates the receiver in thecenter of bore hole 304 while a logging cable 306 of any desired lengthconnects the receiver to the base instrumentation at the ground surface.As is well known, cable 306 may be wound on a reel 308 after passingover pulleys 309 and 310, to permit the receiver to be raised andlowered. Connection between the cable and the base circuitry may bemaintained by way of collector rings 312 and suitable brushes 314 on thereel 308, the brushes 314 being connected to the base portion circuitry316 by way of cable 318. The circuitry included in base portion 316 isillustrated in FIG. 4, with cables 306 and 318, collector rings 312, andbrushes 314 together being the functional equivalent of the connector 52illustrated in FIGS. 3, 4 and 5. The indicators and recorders 320 willprovide readings which permit accurate guidance of a drill 330 locatedin the off-vertical bore 332.

In the present example, bore 332 is to be drilled to connect verticalwell 334 with target well 304, and drill 330 is therefore to be guidedtoward the sensorreceiver 300. Drill 330 may be of any desired type, butconveniently may be a jet type drill having a rotatable nozzle whichcauses a high velocity stream of fluid to be ejected in a selecteddirection. The nozzle may be rotated by means of a motor within thedrill string

1. A highly sensitive magnetometer for sensing and measuring a magneticfield, comprising first and second sensor elements, each sensor elementincluding a saturable core carrying a primary winding and a secondarywinding, and each said secondary winding producing an outputcorresponding to a component of the sensed magnetic field; a source ofalternating current for driving said primary windings with a current ofpredetermined frequency and amplitude; null balance means for each saidsensor element to compensate for undesired output signals and establisha null condition in said magnetometer; detector means including firstand second diodes connected to said secondary windings on said first andsecond sensor elements, respectively, to receive the outputs from saidsecondary windings and produce direct current output signalscorresponding to said components of said sensed magnetic field;unbalancing means for each sensor element including an unbalancingcircuit for feeding into each secondary winding a signal proportional toand synchronous with the alternating drive current applied to thecorresponding primary winding, the amplitude of the current fed to saidsecondary windings being sufficient to overcome the threshold of saiddiodes at said null condition whereby said detector means will respondto very low level outputs from said secondary windings due to very smallsensed magnetic fields; and means for amplifying and indicating saiddetector output signals.
 2. The magnetometer of claim 1, wherein saidunbalancing circuit includes means for connecting the primary winding ofeach sensor to its corresponding secondary winding to feed directly toeach secondary winding a portion of the alternating drive currentapplied to the corresponding primary winding.
 3. The magnetometer ofclaim 2, wherein said components of the sensed magnetic field are vectorcomponents.
 4. The magnetometer of claim 3, wherein said first andsecond sensor elements are mutually perpendicular.
 5. The magnetometerof claim 2, wherein said unbalancing circuit means comprises aresistor-capacitor loop connected between the primary and itscorresponding secondary winding of each sensor element.
 6. Themagnetometer of claim 5, wherein said unbalancing means furthercomprises an unbalanced primary winding on each said sensor elementwhereby said primary winding induces a different flux in differentportions of the sensor core on which it is wound.
 7. The magnetometer ofclaim 1, wherein said means for indicating includes recorder means. 8.The magnetometer of claim 2, wherein said null balance means comprises anull balancing circuit having first and second independently variabledirect current outputs, and means for applying said first and secondnull circuit outputs to said first and second sensor elements,respectively.
 9. The magnetometer of claim 8, wherein said nullbalancing circuit comprises a regulated source of positive and negativedirect current, and potentiometer means connected across said regulatedsource of direct current for selecting the polarity and amplitude ofsaid first and second variable direct current outputs.
 10. Themagnetometer of claim 9, wherein said first and second variable directcurrent outputs are connected to the secondary windings of said firstand second sensor elements, respectively.
 11. The magnetometer of claim10, further including selector switch means for connecting either one orthe other of said variable direct current outputs to its correspondingsecondary windings, whereby a null balance condition will be establishedin only one sensor element at a time.
 12. The magnetometer of claim 11,wherein said selector switch means further includes means for connectingsaid means for amplifying and indicating to the detector meanscorresponding to the selected sensor element, whereby only one sensorelement at a time is used to sense an external magnetic field.
 13. Themagnetometer of claim 10, further including isolating means between saidnull balance means and said detector means for preventing said first andsecond variable direct current outputs from affecting said detectoroutputs.
 14. The magnetometer of claim 1, wherein said null balancemeans comprises first and second sources of variable direct currentconnectable directly to said first and second secondary windings,respectively.
 15. The magnetometer of claim 1, further includingequalizer means connected to each of said first and second diodes forequalizing said output signals.
 16. The magnetometer of claim 1, whereinsaid null balance means includes selector switch means for establishinga null condition in only one or the other of said sensor elementsthereby to activate the selected sensor element, said selector switchfurther including means for connecting the activated sensor element tosaid means for amplifying.
 17. The magnetometer of claim 1, furtherincluding means for operating each said sensor element in resonance. 18.The magnetometer of claim 17, wherein said means for operating eachsensor element in resonance comprises capacitor means connected inparallel with each said primary winding.
 19. The magnetometer of claim18, wherein said null balance means comprises a null balancing circuithaving first and second independently variable direct current outputs,and means for applying said first and second null circuit outputs to thesecondary windings of said first and second sensor elements,respectively.
 20. The magnetometer of claim 19, further includingselector switch means for connecting either one or the other of saidvariable direct current outputs to a corresponding secondary winding,whereby a null balance condition will be established in only theselected sensor element.
 21. The magnetometer of claim 20, wherein saidselector switch further includes means for connecting the selectedsensor element through its corresponding detector means to said meansfor amplifying, whereby said sensor elements may be alternatelyconnected to said means for amplifying.
 22. The magnetometer of claim21, wherein said unbalancing means for each sensor element comprisescircuit means for feeding to each secondary winding a portion of thealternating drive current applied to the corresponding primary winding.23. The magnetometer of claim 22, further including isolating meansbetween said null balance means and said detector means for preventingsaid first and second independently variable direct current outputs fromaffecting said detector outputs.
 24. The magnetometer of claim 1,wherein each said saturable magnetic core comprises a generallyrectangular core element having two parallel legs, said primary windingincluding two coils wound additively on the two parallel legs of saidcore element and said secondary winding including two coils wound inopposition on the said two parallel legs of said core element, wherebyin the absence of external magnetic fields an alternating currentapplied to said primary winding produces substantially no resultantoutput in said second ary winding, an external field producing an outputon said secondary winding which is proportional in amplitude andpolarity to the component of the said external field which is parallelto the said two parallel legs.
 25. The magnetometer of claim 14, whereineach of said secondary windings is connected to a load resistor, saiddetector means being connected across said load resistors.
 26. Themagnetometer of claim 25, wherein said first and second sources ofvariable direct current in said null balance means are connected tocorresponding ones of said load resistors, said magnetometer furtherincluding isolating capacitors between said null balance means and saiddetector means for isolating said detector means from said first andsecond variable direct current sources.
 27. The magnetometer of claim26, wherein said unbalancing circuit means comprises aresistor-capacitor loop connected between the corresponding primary andsecondary windings of each sensor element, the capacitors in said loopsserving to isolate said primary windings from said sources of variabledirect current in said null balance means.
 28. The magnetometer of claim27, wherein said source of alternating drive current is capacitivelycoupled to said primary windings.
 29. The magnetometer of claim 27,further including capacitor means connected across each said primarywinding for operating said sensor elements in resonance.
 30. Themagnetometer of claim 29, further including selector switch means forconnecting only one of said sensor means through its correspondingdetector means to said means for amplifying, said means for amplifyingbeing a direct current amplifier.
 31. The magnetometer of claim 30,further including equalizer means connected to each of said first andsecond diodes for equalizing said output signals before they areamplified by said amplifier.
 32. The magnetometer of claim 31, whereinsaid detector means further includes third and fourth diodes, said firstand third diodes being connected in opposition across the load resistorfor said first sensor to produce a first direct current output signaland said second and fourth diodes being connected in opposition acrossthe load resistor for said second sensor to produce a second directcurrent output signal, said selector switch means serving to connecteither said first or said second direct current output signal to saidamplifier.
 33. The magnetometer of claim 26, wherein said unbalancingmeans further includes an unbalanced primary winding on each saidsaturable magnetic core.
 34. The method of operating a highly sensitivemagnetometer to permit measurement of the strength and direction ofmagnetic fields having a field strength of the order of 0.01 gammas,said magnetometer including first and second sensor elements, each saidsensor element comprising a saturable core member carrying a primarywinding and a secondary winding, comprising the steps of: generating analternating current of determined frequency and amplitude; driving theprimary winding of each said sensor element with said alternatingcurrent and in resonance; generating regulated positive and negativedirect currents; applying to a selected one of said sensor elements aportion of said direct current, the said portion being selectivelyvariable as to polarity and magnitude to establish in the selectedsensor element a null condition, whereby a sensed magnetic field willproduce a measurable change only in the secondary winding of saidselected sensor element; connecting the secondary winding of each sensorto corresponding detector diodes to convert alternating current outputsfrom said sensors due to the effects of sensed magnetic fields intocorresponding direct current outputs; feeding to each secondary windingby way of corresponding resistorcapacitor loops a portion of saidalternating drive current driving the corresponding primary windings,whereby each secondary winding produces a predetermined alternatingoutput signal in said null condition sufficient to overcome thethreshold of conduction for said detector diodes, whereby the desiredoutputs from said sensors due to said sensed magnetic fields are notblocked by said diodes; feeding the direct current outputs from saiddetector diodes through corresponding channels to equalizers to balancesaid channels; and amplifying and measuring the selected output signalto detect the magnitude and direction of said sensed magnetic field.